Ink-jet printing generally involves the controlled delivery of ink drops from an ink-jet pen reservoir to a printing surface. One type of ink-jet printing, known as drop-on-demand printing, employs a pen that includes a print head and ink reservoir. The print head is responsive to control signals for ejecting drops of ink from the ink reservoir.
Drop-on-demand type print heads typically use one of two mechanisms for ejecting drops: thermal bubble or piezoelectric pressure wave. A thermal bubble type print head includes a thin-film resistor that is heated to vaporize a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through a print head orifice.
Piezoelectric pressure wave type print heads use a piezoelectric element that is responsive to a control signal for abruptly compressing a volume of ink in the print head to produce a pressure wave that forces the ink drops through the orifice.
Although conventional drop-on-demand print heads are effective for ejecting or "pumping" ink drops from a pen reservoir, they do not include any mechanism for preventing ink from permeating through the print head when the print head is inactive. Accordingly, drop-on-demand techniques require the fluid in the ink reservoir to be stored in a manner that provides a slight back pressure at the print head to prevent ink leakage from the pen whenever the print head is inactive. As used herein, the term "back pressure" means the partial vacuum within the pen reservoir that resists the flow of ink through the print head. Back pressure is considered in the positive sense so that an increase in back pressure represents an increase in the partial vacuum. Accordingly, back pressure is measured in positive terms, such as inches of water column height.
The back pressure at the print head must be at all times strong enough for preventing ink leakage. The back pressure, however, must not be so strong that the print head is unable to overcome the back pressure to eject ink drops. Moreover, the ink-jet pen must be designed to operate despite environmental changes that cause fluctuations in the back pressure.
A severe environmental change that affects reservoir back pressure occurs during air transport of an ink-jet pen. In this instance, ambient air pressure decreases as the aircraft gains altitude and is depressurized. As ambient air pressure decreases, a correspondingly greater amount of back pressure is needed to keep ink from leaking through the print head. Accordingly, the level of back pressure within the pen must be regulated during times of ambient pressure drop.
The back pressure within an ink-jet pen reservoir is also subjected to what may be termed "operational effects." One significant operational effect occurs as the print head is activated to eject ink drops. The consequent depletion of ink volume from the reservoir increases (creates greater vacuum) the reservoir back pressure. Without regulation of this back pressure increase, the ink-jet pen will eventually fail because the print head will be unable to overcome the increased back pressure to eject ink drops. Such failure wastes ink whenever the failure occurs before all of the useable ink within the reservoir has been ejected.
Past efforts to regulate reservoir back pressure in response to environmental changes and operational effects have included mechanisms that may be collectively referred to as accumulators. Generally, an accumulator comprises a movable mechanism that is mounted to the pen to define an accumulator volume that is in fluid communication with the ink-jet pen reservoir volume. The accumulator moves between a minimum volume position and a maximum volume position in response to changes in the level of the back pressure within the reservoir. Accumulator movement changes the overall volume of the reservoir to regulate back pressure level changes so that the back pressure remains within an operating range that is suitable for preventing ink leakage while permitting the print head to continue ejecting ink drops.
For example, as the difference between ambient pressure and the back pressure within the pen decreases as a result of ambient air pressure drop, the accumulator moves to increase the reservoir volume, thereby to increase the back pressure to a level (within the operating range mentioned above) that prevents ink leakage. Put another way, the increased volume attributable to accumulator movement lessens a reduction in the difference between ambient air pressure and back pressure that would otherwise occur if the reservoir were constrained to a fixed volume as ambient air pressure decreased.
Accumulators also move to decrease the reservoir volume whenever environmental changes or operational effects (for example, ink depletion occurring during operation of the pen) cause an increase in the back pressure. The decreased volume attributable to accumulator movement reduces the back pressure to a level within the operating range, thereby permitting the print head to continue ejecting ink.
Accumulators are usually equipped with internal or external resilient mechanisms that continuously urge the accumulators toward a position for increasing the volume of the reservoir. The effect of the resilient mechanisms is to retain a sufficient minimum back pressure within the reservoir (to prevent ink leakage) even as the accumulator moves to increase or decrease the reservoir volume.
Accumulators have been used in conjunction with devices known as bubble generators. Bubble generators permit ambient air bubbles to enter the ink reservoir once the accumulator has moved to its minimum volume position (that is, once the accumulator is unable to further reduce the back pressure within the reservoir) and the back pressure continues to rise as the print head continues to eject ink from the reservoir. The air bubbles delivered by the bubble generator increase the fluid volume in the reservoir thereby to keep the reservoir back pressure from increasing to a level that would cause failure of the print head.
Bubble generators generally comprise a small-diameter orifice that provides fluid communication between the pen reservoir and ambient air. Because of the reservoir back pressure maintained by the accumulator, ink does not leak through the bubble generator orifice. Moreover, because the reservoir ink normally covers the bubble generator orifice, ambient air is unable to enter the reservoir until the back pressure increases to a level great enough for drawing air bubbles into the reservoir ink.
One problem with the use of bubble generators arises whenever the pen is moved (for example, inverted) to a position where the reservoir ink no longer covers the orifice to restrict the inflow of ambient air. The consequent unrestricted flow of ambient air into the reservoir will eliminate the back pressure that is required for proper operation of the print head.